Evidence of sulfide melting and melt fractionation during amphibolite facies metamorphism of the Rajpura–Dariba polymetallic sulfide ores

Partial melting of sulfide ores during prograde metamorphism could have been more prevalent than generally accepted. However, identification of such melting is difficult as sulfide melts do not form glasses and the textures generated on quenching are obliterated due to the tendency of sulfides for ready recrystallization. The polymetallic base metal sulfide deposit at Rajpura–Dariba, Rajasthan, India is a typical stratiform ore metamorphosed to the middle amphibolite facies. The peak metamorphic temperature of 600°C should have been sufficient to initiate sulfide melting as evident from experimental studies in the ZnS–PbS–Cu2S–FeS2–S system. Further, syn-metamorphic melting of the original SEDEX ore was abetted by the high fS2 condition that prevailed as a consequence of barite dissolution. A Zn–Fe–S melt containing minor Pb, Sb and Cu but no Ag fractionated from an initial melt in the above system resulting in a residual immiscible sulfosalt-bearing PbS melt. The final metallic melts, represented by formation of dyscrasite (Ag3Sb) from the sulfosalt-bearing melt and breithauptite (NiSb) or ullmannite (NiSbS) from the sulfosalt-absent melt, were a product of independent fractional crystallization of the immiscible sulfide and PbS–sulfosalt melts.